Add C++ filter example

The filter example uses the C++ template which helps new filter creation
by showing the minimal implementation required for a filter.

examples/examplefilter.cc

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+// All log messages from this module are prefixed with this
+#define MXS_MODULE_NAME "examplefilter"
+
+#include "examplefilter.hh"
+
+// This declares a module in MaxScale
+extern "C" MXS_MODULE* MXS_CREATE_MODULE()
+{
+    static MXS_MODULE info =
+    {
+        MXS_MODULE_API_FILTER,
+        MXS_MODULE_IN_DEVELOPMENT,
+        MXS_FILTER_VERSION,
+        "An example filter that does nothing",
+        "V1.0.0",
+        RCAP_TYPE_NONE,
+        &ExampleFilter::s_object, // This is defined in the MaxScale filter template
+        NULL, /* Process init. */
+        NULL, /* Process finish. */
+        NULL, /* Thread init. */
+        NULL, /* Thread finish. */
+        {
+            { "an_example_parameter", MXS_MODULE_PARAM_STRING, "a-default-value" },
+            { MXS_END_MODULE_PARAMS }
+        }
+    };
+
+    return &info;
+}
+
+ExampleFilter::ExampleFilter()
+{
+}
+
+ExampleFilter::~ExampleFilter()
+{
+}
+
+// static
+ExampleFilter* ExampleFilter::create(const char* zName, char** pzOptions, MXS_CONFIG_PARAMETER* pParams)
+{
+    return new ExampleFilter();
+}
+
+ExampleFilterSession* ExampleFilter::newSession(MXS_SESSION* pSession)
+{
+    return ExampleFilterSession::create(pSession, this);
+}
+
+// static
+void ExampleFilter::diagnostics(DCB* pDcb) const
+{
+}
+
+// static
+json_t* ExampleFilter::diagnostics_json() const
+{
+    return NULL;
+}
+
+// static
+uint64_t ExampleFilter::getCapabilities()
+{
+    return RCAP_TYPE_NONE;
+}